At a recent American Academy of Orthopedic Surgeons (AAOS) conference, two independent studies established a link between fractures and low blood levels of the vitamin.

Vitamin D is essential for the body to properly absorb calcium, so the link between low levels with weaker bones and increased risk of fractures makes sense.

In one of the studies presented to the AAOS, 889 adults treated for fracture at a U.S. trauma center were tested for blood levels of vitamin D. Seventy-eight percent of patients were found to have insufficient levels of the critical vitamin and 39% were found to be deficient.

A second South Korean study found that among 104 post-menopausal women with wrist fractures, blood levels of vitamin D were significantly lower than those of age-matched controls with no fracture.

Several studies have raised the issue about the protective effects of vitamin D on other disorders, including cancer, heart disease, high blood pressure, and autoimmune diseases such as rheumatoid arthritis.

Laboratory studies and limited human research suggest that vitamin D has anti-cancer activity and plays a role in asthma, type 2 diabetes, multiple sclerosis, pre-eclampsia, depression, autism, and memory loss.However, there are no large, controlled clinical trials that have firmly established the link between vitamin D levels and these various disorders. Furthermore, there’s no proof that vitamin D supplementation will reverse these conditions.

The fat-soluble vitamin is naturally present in a few foods. Fish sources such as swordfish, salmon, sardines and tuna provide the greatest concentrations of vitamin D. The vitamin is added to other staples such as orange juice, milk, yogurt, and margarine. Liver and eggs are other good sources.

Much of our vitamin D comes from sun exposure, where the skin triggers vitamin D synthesis. Dark-skinned persons may need to consider additional supplementation, because their natural pigment may block UV stimulation of the vitamin D synthesis pathway. This may help to explain, in part, the increased incidence of such vitamin D deficiency-linked conditions as high blood pressure, kidney disease, heart disease and colorectal cancer.

Cells throughout the body respond to vitamin D through special receptors that detect its presence. Much like a hormone, the vitamin affects the function of more than 200 genes in diverse tissues, including sex organs, skin, heart, brain, breasts and intestines.

The Institute of Medicine considers a blood level of 20 nanograms per milliliter the minimum acceptable level. But many endocrine and bone specialists would prefer levels of 30 nanograms or more.

In response to the call for large clinical trials, the National Institutes of Health is sponsoring a placebo-controlled trial called Vital involving 20,000 men over 50 and women over 55.To be completed in 2016, the trial will assess the effect of daily supplement of 2,000 I.U. of vitamin D on the risk of developing heart disease, cancer and stroke.

Until such results are available, the Endocrine Society recommends that people at risk for vitamin D deficiency have their blood serum levels checked. People with osteoporosis, kidney disease, liver failure, various malabsorption disorders, and those taking certain medications that can interfere with vitamin D absorption or synthesis are considered at risk.

If you have low levels of vitamin D, talk to your doctor about supplementation.